The invention relates to improvements in apparatus for preventing changes of the positions of machines, or of one or more units or parts of machines, especially measuring machines wherein a mobile part or unit is displaceable relative to another unit or part along one or more predetermined paths.
Apparatus of the type to which the present invention pertains can employ bearings which support a first part of the machine and can apply to the first part forces of variable magnitude in order to counteract the tendency of the first part to leave its position as a result of movement of a second part relative to the first part. The bearings can cause the first part to "float" above the floor or above the ground and are adjustable so as to offer a greater or a less pronounced resistance to changes in orientation and/or to other changes in the position of the first part, for example, as a result of movement of the second part relative to the first part whereby the center of gravity of the machine including the first and second parts moves to a different position and the first part tends to change its inclination relative to the floor or ground.
In a presently known apparatus for controlling the position of a machine table which is fixedly installed in the frame of the measuring machine, the position of the table is numerically controlled to thus ensure that movements of a carrier for a measuring instrument relative to a workpiece on the table do not entail a change of position of the table and of the machine frame relative to the floor. The apparatus employs a digital computer which transmits signals for initiation of movements of the carrier and its measuring instrument relative to the table and the workpiece. Reference may be had to the German-language publication "Konstruktion 38" (1986), Volume 4, pages 139-148. U.S. Pat. No. 3,831,283 to Pagella et al. discloses a machine which has a very large mass and is mounted on pneumatic bearings each of which constitutes a cylinder and piston arrangement. Such bearings are intended to prevent the machine from performing movements in response to vibrations of the floor which are caused by neighboring machines, by vehicles which are driven along the floor and/or by attendants walking on the floor. The carrier resembles a gantry and its movements relative to the work-supporting table entail a change of inclination of the table as a result of shifting of the center of gravity of the machine and/or as a result of acceleration forces acting upon the gantry. Such changes in inclination of the table can affect the accuracy of measurement, especially if the measurement must be carried out with minute tolerances in the range of one or more microns.
In order to compensate for changes in the position of the machine table and the workpiece as a result of movements of the gantry and of the measuring instrument relative to the table, the adjusting apparatus of such machine comprises closed loop position control circuits each of which includes one of the bearings. Each circuit ascertains the momentary position of the machine and compares such position with a desired or reference position. If the monitored position deviates from the desired position, the circuit adjusts the respective bearing so as to eliminate the difference between the two positions. A drawback of such adjustment is that unavoidable inertia of the position control circuits prevents the corresponding bearings from immediately reacting to ascertained changes in position of the machine. In other words, the adjustment is too late because it normally takes place after the position control circuit already assumes a steady state condition. In fact, it is even possible that the delayed adjustment of a bearing imparts to the machine a growing vibratory movement, for example, if the adjustment of a bearing is delayed sufficiently to enable the machine to reassume its proper position as a result of further advancement of the gantry with the measuring instrument relative to the table and the workpiece thereon. A proposal to avoid such problems with conventional measuring machines involves a pronounced reduction of speed so that the measurement of a workpiece takes up a relatively long interval of time. The problems are analogous if the table and the workpiece thereon are moved relative to the carrier of the measuring instrument.
Certain other conventional measuring machines are disclosed in German Offenlegungsschrift No. 24 20 928 of Yamaji et al. and in German Offenlegungsschrift No. 25 33 479 of Petersen et al.